Abstract

The fabrication of NanoTube Black, a Vertically Aligned carbon NanoTube Array (VANTA) on aluminium substrates is reported for the first time. The coating on aluminium was realised using a process that employs top down thermal radiation to assist growth, enabling deposition at temperatures below the substrate’s melting point. The NanoTube Black coatings were shown to exhibit directional hemispherical reflectance values of typically less than 1% across wavelengths in the 2.5 µm to 15 µm range. VANTA-coated aluminium substrates were subjected to space qualification testing (mass loss, outgassing, shock, vibration and temperature cycling) before their optical properties were re-assessed. Within measurement uncertainty, no changes to hemispherical reflectance were detected, confirming that NanoTube Black coatings on aluminium are good candidates for Earth Observation (EO) applications.

Out-of-plane axis random vibration response (solid blue line) for a mass dummy during jig tailoring exercise. This is the profile which the blackened coupons were subsequently subjected to. Solid red line shows the response from analysis of flight hardware when subjected to random vibration in out-of-plane axis; this profile was the target for coupon vibration.

Table 4 Summary of Some of the Conditions Which a Black Coating Has to Satisfy Before it is Adopted as a Blackbody Cavity Coating for EO Applications, Together with Comments Indicating How Well NanoTube Black Coatings Satisfy Each Condition

Mass loss and outgassing test results (highest values taken)

Table 2

Shock response spectrum requirement (tolerance + 6dB −6dB)

Table 3

TV cycling limits

Maximum temperature limit

Minimum temperature limit

Number of cycles

Dwell time

Temperature rate of change

+ 100°CTolerance −0/+3°C

−100°CTolerance −3/+0°C

6

2 hours

<10 K min−1

Table 4

Summary of Some of the Conditions Which a Black Coating Has to Satisfy Before it is Adopted as a Blackbody Cavity Coating for EO Applications, Together with Comments Indicating How Well NanoTube Black Coatings Satisfy Each Condition

Condition

Suitability of NanoTube Black

The coating must have high thermal conductivity to ensure that the temperature at the “air” (vacuum) side of the coating does not differ significantly from the temperature of the substrate.

This is required in order to ensure that the temperature drop across the black coating is small. NanoTube Black has excellent thermal conductivity across the thickness of the coating - carbon nanotubes have been shown to have very high thermal conductivity along their length, typically 6600 W m−1 K−1 . VANTA coatings have also been shown to have high thermal conductivity transverse to the nanotube forest [26].

Good thermal stability and repeatable optical performance after thermal cycling

The samples were subjected to a number of temperature cycles between −100 °C and 100 °C without any observed degradation. Further tests involving immersion in liquid nitrogen (−196 °C) also showed no degradation.

Resistant to the launch vibration and staging shock.

Vibration and shock tests carried out in the current study demonstrate that the coating satisfies this requirement.

Resistance to chemical attack such as the atomic oxygen encountered in low earth orbit, as well as resistance to moisture.

This was not investigated in the current study.

The coating must have low outgassing.

Outgassing tests carried out in the current study confirm that NanoTube Black coatings satisfy this requirement.

The coating must have low “particle generation” i.e. fragments of the black coating must not be dislodged, thus altering the characteristics of the black coating and depositing on nearby optical surfaces, causing their degradation.

There was no significant loss of mass of the coatings after the samples were subjected to the vibration and shock tests, confirming that NanoTube Black coatings satisfy this requirement.

Easy to fabricate on complex geometries

Areas up to 200 mm by 200 mm can currently be coated. The growth of VANTA coatings on curved surfaces/3D objects was not investigated in the current study.

Manufacturing repeatability.

Unlike chemical etching, the procedure used to grow NanoTube Black is very controlled and the results are repeatable.

Long-term storage/stability/durability.

No long term degradation test have so far been carried out but the nature of NanoTube Black coatings implies good long term stability, unlike some other black coatings such as gold-black whose structure is known to collapse under certain storage conditions.

Difficulties in acquiring ITAR-controlled materials are overcome with the development of the NanoTube Black coating.

Cost (less critical for space applications).

The superior absorbance of NanoTube black allows much smaller blackbody cavities than other black finishes, meaning that a higher per-unit-area cost for NanoTube black is offset by a smaller area to be coated.

Ideally the coating must be able to coat any substrate.

So far VANTA coatings have been grown on a variety of substrates and there is no reason to prevent its deposition on stable substrates whose melting point is above 450 °C.

Tables (4)

Table 1

Mass loss and outgassing test results (highest values taken)

TML = 0.003%

RML = 0.01%

CVCM = 0.00%

Table 2

Shock response spectrum requirement (tolerance + 6dB −6dB)

Frequency (Hz)

Acceleration (g)

100

20

1000

2000

10000

2000

Table 3

TV cycling limits

Maximum temperature limit

Minimum temperature limit

Number of cycles

Dwell time

Temperature rate of change

+ 100°CTolerance −0/+3°C

−100°CTolerance −3/+0°C

6

2 hours

<10 K min−1

Table 4

Summary of Some of the Conditions Which a Black Coating Has to Satisfy Before it is Adopted as a Blackbody Cavity Coating for EO Applications, Together with Comments Indicating How Well NanoTube Black Coatings Satisfy Each Condition

Condition

Suitability of NanoTube Black

The coating must have high thermal conductivity to ensure that the temperature at the “air” (vacuum) side of the coating does not differ significantly from the temperature of the substrate.

This is required in order to ensure that the temperature drop across the black coating is small. NanoTube Black has excellent thermal conductivity across the thickness of the coating - carbon nanotubes have been shown to have very high thermal conductivity along their length, typically 6600 W m−1 K−1 . VANTA coatings have also been shown to have high thermal conductivity transverse to the nanotube forest [26].

Good thermal stability and repeatable optical performance after thermal cycling

The samples were subjected to a number of temperature cycles between −100 °C and 100 °C without any observed degradation. Further tests involving immersion in liquid nitrogen (−196 °C) also showed no degradation.

Resistant to the launch vibration and staging shock.

Vibration and shock tests carried out in the current study demonstrate that the coating satisfies this requirement.

Resistance to chemical attack such as the atomic oxygen encountered in low earth orbit, as well as resistance to moisture.

This was not investigated in the current study.

The coating must have low outgassing.

Outgassing tests carried out in the current study confirm that NanoTube Black coatings satisfy this requirement.

The coating must have low “particle generation” i.e. fragments of the black coating must not be dislodged, thus altering the characteristics of the black coating and depositing on nearby optical surfaces, causing their degradation.

There was no significant loss of mass of the coatings after the samples were subjected to the vibration and shock tests, confirming that NanoTube Black coatings satisfy this requirement.

Easy to fabricate on complex geometries

Areas up to 200 mm by 200 mm can currently be coated. The growth of VANTA coatings on curved surfaces/3D objects was not investigated in the current study.

Manufacturing repeatability.

Unlike chemical etching, the procedure used to grow NanoTube Black is very controlled and the results are repeatable.

Long-term storage/stability/durability.

No long term degradation test have so far been carried out but the nature of NanoTube Black coatings implies good long term stability, unlike some other black coatings such as gold-black whose structure is known to collapse under certain storage conditions.

Difficulties in acquiring ITAR-controlled materials are overcome with the development of the NanoTube Black coating.

Cost (less critical for space applications).

The superior absorbance of NanoTube black allows much smaller blackbody cavities than other black finishes, meaning that a higher per-unit-area cost for NanoTube black is offset by a smaller area to be coated.

Ideally the coating must be able to coat any substrate.

So far VANTA coatings have been grown on a variety of substrates and there is no reason to prevent its deposition on stable substrates whose melting point is above 450 °C.